Three cohorts comprising 17,500 clean-up workers from Estonia, Latvia, and Lithuania who were sent to Chernobyl (Ukraine) following the reactor accident in 1986 are being monitored for cancer incidence and mortality. Documented doses were low (about 11 cGy on average) and an excess of leukemia could not be detected in this population at this time. Results of thyroid screening examinations on approximately 2,000 workers revealed no association for thyroid nodules with radiation exposure. This investigation is being expanded to include 80,000 clean-up workers from the Ukraine. Cancer mortality is being evaluated in a cohort study of 20,000 nuclear workers chronically exposed to large doses of external radiation and/or plutonium at the Mayak nuclear weapons plant in the South Ural region in Russia. Results will be compared with those from studies of American nuclear workers exposed to low levels of radiation. A follow-up study of cancer risk in 145,000 x-ray technologists is ongoing. A nested case-control study of breast cancer found no association with number of years worked in the medical radiation field or with specific work practices. Mutations in the BRCA1 gene were identified in 3 of 53 women diagnosed with early-onset (before age 35) breast cancer; no mutations were observed among 17 women with early-onset ovarian cancer. Chromosome aberrations in circulating lymphocytes, identified using flourescent in-situ hybridization (FISH), correlated closely with number of years worked but not with documented doses from the nation's largest commercial dosimetry company, Landauer Inc. Film badge readings, available for 90,000 technologists from Landauer, will be used in conjunction with FISH results and work history data to reconstruct dose estimates for individual technologists.